Solid particulate material bins can be categorized as process bins and storage bins. Process bins are employed in continuous or batch type processes and hold material, typicaly, for relatively short time periods of operation of the process. Storage bins are employed to hold material for relatively longer time periods, and frequently are not directly involved in a process other than to supply material to or receive material from the processing apparatus. Storage bins are usually considerably larger than process bins and, in the case of grains and other raw materials, may have a capacity measured in railcar loads.
Two types of physical segregation occur in both storage and process bins that can degrade the quality of the final product or cause difficulties in the processing of the material. The first of these types of secregation results from the non-uniform witdrawal of the material from the bin. In the case of a hopper-bottom bin with a single centrally located outlet for the material, the fine fractions thereof tend to concentrate toward the center and the coarse fractions concentrate near the walls. As a result, even if the material was homogenous when loaded into the bin, the process of withdrawing it can concentrate the different fractions of materials such that the consistency of the material varies as it is withdrawn from the bin.
This form of segregation can be reduced or eliminated by use of the system for controlling such segregation described in the inventor's U.S. Pat. No. 4,030,633. The solid particulate material blender described in the inventor's U.S. Pat. No. 3,571,321 may also be used to combat this form of segregation.
The second type of segregation that tends to occur in bulk material handling is that caused by non-uniformity in the input material. In the chemical industry, such variations in consistency may be the result of differences in the purity, dryness, or partical size of batches loaded sequentially into the hopper. In the grain industry, the moisture, protein and screenings content of the grain frequently varies from carload to carload. Layers or phases of segregated, nonuniform material thus are frequently created in bulk material by virtue of its serial loading into the bin. The two types of segregation generally result in the output of a nonuniform product from the bin.
In the grain milling industry, such nonuniformity of the grain which results from this segregation is particularly critical. If the millability of the grain changes as it is withdrawn from the bin, the milling equipment must be constantly adjusted to compensate for such changes. This increases the labor and expense of milling the grain. If the milling equipment is not properly monitored, the flour produced can be of uneven quality which can in turn affect the quality of goods made therewith. The problems caused by non-uniformity between lots of material in a bin may be aggravated if the material is withdrawn such that there is an abrupt transition between withdrawal of material from a first lot and withdrawal from a second lot. In the case of wheat, such a transition may result in an abrupt change in protein or moisture content of wheat in the stream of material withdrawn from the bin.
In order to mitigate the problems associated with nonuniformity of grain, many mills have instituted "master mix" programs to blend and mix the grain prior to milling. Such programs are generally expensive, however, as they require considerable handling of the grain and require considerable excess storage capacity. A "master mix" program might be implemented as follows.
Grain from five sources is first matched to yield desired protein quality and quantity. The grain is simultaneously drawn from the five sources and loaded sequentially into three silos. The grain is then simultaneously withdrawn from these three silos and again sequentially loaded into three additional silos. This process of simultaneous unloading and sequential loading is again repeated, after which the grain is drawn off simultaneously once more and loaded sequentially into three silos until the first silo is completely full and the second is half full. At this point, grain is drawn simultaneously from the first and second silos and transported to the mill for processing. The third silo is filled as the first and second silos are emptied. When the second silo has been emptied, grain is withdrawn from the first and third silos simultaneously.
As is apparent from the above description, "master mix" programs result in dilution of localized nonuniformities in the grain finally delivered to the mill and attempt to deal with the problem of aggregation of material by density and size. Such programs thus improve the uniformity of the grain delivered to the mill and thereby decrease the need for attention to the milling machinery.
In the plastics industry, plastic materials are frequently furnished to processors in particulate form. It is also important in this industry that the material withdrawn from bins for processing be uniform in nature and that levels of impurities be maintained below a predetermined maximum value. In addition, small quantities of off-grade material are occasionally salvaged by mixing them with material which has a lower concentration of impurities than the maximum acceptable value. This off-grade material may be loaded into a bin to form a thin layer or spike. It is thus common to blend the plastic material prior to processing to ensure that it is of the desired homogeneity. Such mixing is frequently accomplished by recirculating the material through a blending device. This mixing of course, is expensive in terms of energy usage, manpower and equipment.